The disclosure of Japanese Patent Application No. HEI 11-247342 filed on Sep. 1, 1999 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to a hydrogen storage device and a hydrogen storage system employing a hydrogen occlusion alloy.
2. Description of the Related Art
As a general art for discharging hydrogen occluded by a hydrogen occlusion alloy, it is known to spread heat generated by a heating element over the entire hydrogen occlusion alloy by means of heat conduction. A total heat capacity in this art is the sum of a heat capacity of the heating element and a heat capacity of the hydrogen occlusion alloy. Thus, part of the energy applied to the hydrogen occlusion alloy for discharge of hydrogen is consumed to heat the heating element. Hence, it takes a long time to heat the hydrogen occlusion alloy, and it is impossible to achieve high energy efficiency.
There is also known an art wherein powder of a hydrogen occlusion alloy encapsulated in a container is directly supplied with electric current to rapidly discharge hydrogen occluded by the hydrogen occlusion alloy and wherein the hydrogen occlusion alloy itself generates heat. In this art, since the hydrogen occlusion alloy itself functions as a heating element for generating heat because of the electrical resistance of the hydrogen occlusion alloy, there is no need to separately provide a heating element. The total heat capacity of the entire system is a heat capacity of the hydrogen occlusion alloy. Also, since the hydrogen occlusion alloy itself generates heat, it is theoretically possible to more rapidly discharge hydrogen in comparison with the case where the entire hydrogen occlusion alloy is heated by means of heat conduction.
However, in the case where powder of a hydrogen occlusion alloy is used, the hydrogen occlusion alloy is fatigued by the repeated absorption and discharge of hydrogen and may become even finer. This leads to a tendency for (pieces of) the hydrogen occlusion alloy that has become finer to scatter in the container and occupy a bottom portion of the container with high density (compaction). In such a case, there is caused a problem of inconsistent density distribution of powder in the container, leading to localization of electric current paths. That is, in general, the hydrogen occlusion alloy that has become finer is localized in the bottom portion of the container. This leads to a decrease in resistance of the bottom portion of the container and thus causes localization of the electric current paths in the bottom portion of the container. In such a state, although the hydrogen occlusion alloy in the bottom portion of the container generates heat by itself, the hydrogen occlusion alloy in other portions decreases in conductivity and thus is mainly heated only by heat conduction from heat-generating portions. For this reason it takes longer to heat the entire hydrogen occlusion alloy. Thus, there is caused a problem of difficulty in rapidly discharging hydrogen, especially when the hydrogen storage device is operating in cold conditions.
Because the hydrogen occlusion alloy that has become finer is localized in the bottom portion of the container, an unexpected load resulting from volume expansion of the hydrogen occlusion alloy is applied to the bottom portion of the container at the time of occlusion of hydrogen. This leads to a problem of damage to the container, such as cracking.
Furthermore, if the hydrogen occlusion alloy has undergone volume changes in accordance with absorption of hydrogen by the hydrogen occlusion alloy or discharge of hydrogen from the hydrogen occlusion alloy, there occurs a change in contact resistance. This affects an electrical resistance between the electrode terminals. Thus, there is caused a problem of difficulty in controlling a temperature of the hydrogen occlusion alloy through supply of electric current. In the case of powder, since the hydrogen occlusion alloy is not constant in shape, it is difficult to constantly maintain physical contact between the electrode terminals and the hydrogen occlusion alloy.
The internal resistance of the hydrogen storage device, namely, the internal resistance of the hydrogen occlusion alloy, is constant. Thus, under a circumstance where the internal resistance of a power source changes, the amount of heat generated by the hydrogen occlusion alloy is not always at its maximum. Thus, in some cases, it is difficult to heat the hydrogen occlusion alloy within an extremely short period of time, and it is impossible to rapidly discharge hydrogen.
The invention has been made in an effort to solve the aforementioned problems. It is an object of the invention to prevent localization of hydrogen occlusion alloy and realize rapid discharge of hydrogen.
In order to achieve the aforementioned object, according to an aspect of the invention, there is provided a hydrogen storage device comprising a plurality of molded pieces made of powder of a hydrogen occlusion alloy and a temperature control system that controls temperatures of the molded pieces. The molded pieces are held within a container portion of the hydrogen storage device.
The aforementioned aspect of the invention makes it possible to prevent localization of the hydrogen occlusion alloy by using the molded pieces and to realize rapid discharge of hydrogen by controlling temperatures of the molded pieces.
In another aspect of the invention, there is provided a hydrogen storage system wherein a plurality of hydrogen storage devices of the aforementioned aspect are connected, comprising a power source for supplying the hydrogen storage devices with electric current and a resistance controller for making a total internal resistance of the molded pieces incorporated in the hydrogen storage devices approximately equal to an internal resistance of the power source. This makes it possible to maximize an amount of heat generated by the molded pieces made of a hydrogen occlusion alloy even if the internal resistance of the power source fluctuates.